The cars, known as DOT-111s, were involved in derailments of oil trains in Casselton, N.D., and Lac-Megantic, Quebec, just across the U.S. border, NTSB member Robert Sumwalt told a House Transportation subcommittee hearing.

The United States on Wednesday warned Russia against a military intervention in Ukraine, saying such a move would be a "grave mistake," as troops in western Russia were placed on high alert for massive new war games in the area, including near the Russian-Ukrainian border.